The CD19 antigen plays an important role in clinical oncology. In normal cells, it is the most ubiquitously expressed protein in the B lymphocyte lineage. CD19 expression is induced at the point of B lineage commitment during the differentiation of the hematopoietic stem cell, and its expression continues through preB and mature B cell differentiation until it is finally down-regulated during terminal differentiation into plasma cells. CD19 expression is maintained in B-lineage cells that have undergone neoplastic transformation, and therefore CD19 is useful in diagnosis of leukemias and lymphomas using monoclonal antibodies (mAbs) and flow cytometry.